Injection molding apparatus for making structural foam plastic is shown in German Pat. No. 1,932,437 in which an inter-connecting passage is surrounded by heating coils for heating up the plastic mass. Intense heating is achieved by use of an inner body which is also equipped with heating coils and so enlarges the heating surface available to the plastic masses. To make the product homogenous, the plastic flow takes place through jets or the cross-section of the flow area is of meander shape. In practice, it has been shown that a more intense heating is desirable; also, with respect to homogenizing, further mixing is desirable. Furthermore, experience has proved that meander-shaped flow passages are expensive because of their complicated design; in addition, jet passages require a high pressure differential to produce flow of the plastic mass. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.
It is, therefore, an outstanding object of the invention to provide injection molding apparatus in which relatively simple parts are used, so that, without large pressure-drop and influence on mixing, intense heating is possible.